We have vernacular phrases being quoted hither and thither from 900 years ago.

We have vernacular texts that we kinda sorta date from 900 years ago, but their language is probably closer to what the scribes wrote in the copies we have, which is nearer to 700 years ago.

So pinning down the vernacular from 1000 years ago is tricky.

The closest I’ll mention here is this snippet of a song about Alexios I Komnenos escaping a conspiracy from 1031, recorded (with much embarrassment about the vulgarity of the language) by his daughter Anna Comnena in her Alexiad:

Quite close; and for all that I changed two words, the first word (εννόησές) could still be used. The second word (υπάγω > ύπα) is still used, but its conjugation has changed, so people wouldn’t understand it.

The only phoneme to have changed since was υ (and οι), switching from /y/ to /i/. (So ύπα for “go!” was /ypa/.) In fact, we have a poem from 1030, making fun of a hillbilly priest pronouncing upsilon as /i/: so the modern pronunciation was already around at the time of the song snippet.

The only substantive difference, really, is all the final n’s being dropped. My guess: Modern Greek speakers would be reminded of Cypriot, which is phonologically conservative. (It also keeps double consonants; I have no idea when they disappeared from the rest of Greek, but I suspect it was later.)

Oh, that song snippet? Anna Comnena was not going to leave it alone in such vulgar garb. She appends a translation into something more decent:

On the Saturday with the name of cheese, much commendation for your sagacity, Alexius. And on the day of Monday after the Sunday, just like some high-flying hawk, you have flown away from the barbarians who meant you harm.